1. Field of the Invention
The present invention relates to fabrication of semiconductor circuit devices. More particularly, the present invention is directed to self-aligned silicide structures and methods of forming the same without excessive consumption of underlying silicon.
2. Description of Related Art
As is well known in processing integrated circuits, electrical contacts must be made among circuit nodes, such as isolated device active regions formed within a single-crystal silicon substrate. As the contact dimensions of devices become smaller, the contact resistance and the sheet resistance of the contacts increase. In this regard, refractory metal suicides have been used for local interconnections to provide low resistance electrical contacts between device active regions within the silicon substrate.
One common method of forming metal silicides is a self-aligned silicide process, often referred to as salicidation. A thin layer of refractory metal, such as titanium, is deposited over a dielectric area and through contact openings formed on the dielectric area to contact underlying silicon circuit elements, such as source and drain active regions formed within a silicon substrate. The structure is generally annealed to form a silicide, such as titanium silicide (predominantly TiSi.sub.2) at a high temperature. During the anneal, the deposited titanium reacts with the silicon in the contact to form TiSi.sub.2 at the contact openings. Titanium which overlies the dielectric area does not form TiSi.sub.2, as the titanium does not contact any silicon. The process is referred to as "self-aligned" because the silicide is formed only where the metal layer contacts silicon, for example, through the contact openings. After the first annealing, the unreacted titanium may be removed in a wet etch, and a post-silicidation anneal is performed to lower the sheet resistance of the silicide to acceptable levels. The final annealing converts titanium silicide from the C49 phase to the lower resistance C54 phase. This self-aligned silicide is often referred to by the short form "salicide."
In the salicidation process, silicon from the contact regions of the substrate diffuses upward into the titanium layer. Similarly, titanium diffuses into the underlying active areas of the silicon substrate. Titanium and silicon react with each other to form a silicide thick enough to provide low sheet resistance. As a result, the doped active area of the silicon substrate becomes thinner due to the consumption of silicon during the reaction. The resultant silicide is said to intrude or sink into the substrate. Over-consumption of the underlying silicon can be problematic for any silicon circuit element, tending to cause voids, and thus device failures. Where contact is made to a shallow junction active area of a silicon substrate, salicide contacts of sufficient thickness cannot be formed without completely destroying a junction.
A need, therefore, exists for an interconnect and method of fabricating the same, which provides the advantages of salicide interconnects without excessive consumption of underlying silicon to which contact is made.